Superior thermoelectric properties of ternary chalcogenides CsAg₅Q₃ (Q = Te, Se) predicted using first-principles...

Tailoring novel thermoelectric materials (TEMs) with a high efficiency is challenging due to the difficulty in realizing both low thermal conductivity and high thermopower factor. In this work, we propose ternary chalcogenides CsAg₅Q₃ (Q = Te, Se) as promising TEMs based on first-principles calculations of their thermoelectric properties. Using lattice dynamics calculations within self-consistent phonon theory, we predict their ultralow lattice thermal conductivities below 0.27 W m^-1 K^-1, revealing the strong lattice anharmonicity and rattling vibrations of Ag atoms as the main origination. By using the mBJ exchange-correlation functional, we calculate the electronic structures with the direct band gaps in good agreement with experiments, and evaluate the charge carrier lifetime as a function of temperature within the deformation potential theory. Our calculations to solve Boltzmann transport equations demonstrate high thermopower factors of 2.5 mW m^-1 K^-2 upon p-type doping at 300 K, comparable to the conventional dichalcogenide thermoelectric GeTe. With these ultralow thermal conductivities and high thermopower factors, we determine a relatively high thermoelectric figure of merit ZT along the z-axis, finding the maximum value of ZT to be 2.5 at 700 K for CsAg₅Se₃ by optimizing the hole concentration. Our computational results highlight the great potentiality of CsAg₅Q₃ (Q = Te, Se) for high-performance thermoelectric devices operating at room temperature.